Since reactions of hydrocarbons such as oxidation, ammoxidation or oxidative dehydrogenation, etc., generally generate a large amount of heat of reaction, a fluidized bed from which heat is easily removed is often used for these reactions. In a fluidized reaction, in order to effectively carry out the reaction while keeping the catalyst particles in a good fluidized state, the catalyst should have, of course, good activity and high strength. In addition, it is very important for the catalyst to have a suitable particle size distribution. However, it is inevitable to some degree that the activity of the catalyst is deteriorated by use for a long period of time or if the reaction takes place under unsuitable conditions. In a fluidized reaction, the catalyst particles of a small particle size are scattered during the reaction even if a catalyst having a suitable particle size distribution is used and, consequently, the particle size distribution of the catalyst tends towards a greater particle size with the fluidized state being deteriorated, by which reaction efficiency is reduced.
To counteract these problems, processes to activate catalysts having a reduced activity are described in U.S. Pat. No. 4,049,575 wherein an antimony containing catalyst is calcined after impregnating with or spraying a solution containing catalyst active components, in U.S. Pat. No. 4,208,303 wherein an iron-antimony catalyst is calcined at a specified temperature range under a non-reducing atmosphere, in U.S. patent application Ser. No. 959,810, filed Nov. 13, 1978, now abandoned, wherein the catalyst is calcined after impregnation with or spraying an aqueous solution of nitric acid or nitrate, in British Pat. No. 1,365,096 wherein an antimony-uranium oxide catalyst complex is heated in a non-reducing gas at a temperature of 800.degree. F.-1800.degree. F., in U.S. Pat. No. 4,052,332 wherein a molybdenum catalyst is calcined after impregnation with molybdenum and bismuth, and in U.S. Pat. No. 4,052,333 wherein a molybdenum catalyst is calcined in a steam-air atmosphere at a temperature of 500.degree.-550.degree. C. However, according to these processes, it is not possible to prevent a reduction in reaction efficiency which occurs as a result of a coarsening of the distribution of particle size. Therefore, it has been proposed to effectively produce a catalyst having a small particle size which is then added to the catalyst in the reactor. A process for producing an antimony oxide containing fluid catalyst described in U.S. Pat. No. 4,107,085 is a typical example, wherein the catalyst after spray drying during production is classified and fractions having an undesirable particle size are crushed and circulated to the step prior to spray drying to prepare a catalyst having a desired distribution of particle size. However, if a stricter distribution of particle size of the catalyst to be added is required, it is not only necessary to use stricter spray drying conditions, stricter classifying conditions or stricter crushing conditions in the production of the catalyst but also productivity of the catalyst deteriorates increasing the production cost thereof.
On the other hand, in order to adjust the particle size distribution of the catalyst, industrially a process which comprises taking out the used catalyst from a reactor, removing coarse particles by classification to adjust the distribution of particle size of the catalyst so that it is in a suitable range, and returning the resulting catalyst to the reactor has been used, In this case, however, the removed coarse particles do not have any use. Further, the amount of the catalyst removed is sometimes more than about 1/3 of the catalyst. Accordingly, leaving the coarse catalyst as it is not economical because a great loss arises.
Further, in producing a fluid catalyst, there is a problem how to dispose of the catalyst, when a catalyst having a large particle size which can not be put to practical use is produced because of the use of unsuitable conditions.
The present inventors have developed previously a process for producing an antimony oxide containing fluid catalyst and such is described in Japanese Patent Publication No. 15471/78. Specifically, this process comprises crushing a catalyst which could not be advantageously used for some reason or other to produce particles of a particle size of about 20.mu. or less and mixing the resulting crushed catalyst with a slurry in a suitable step prior to spray drying during production of the fluid catalyst according to the process described in U.S. Pat. No. 3,657,155 and 3,686,138 by the present inventors so that 50% by weight or less, based on the finished catalyst, is used. However, this process is not always satisfactory because of problems in that a large amount of energy is required for finely crushing the catalyst because the catalyst to be crushed was calcined at a high temperature and has high strength, that equipments such as slurry pumps or nozzles, etc. are extensively abraded in spray drying of a slurry containing the finely crushed catalyst and, consequently, the frequency in replacement of parts of equipment increases, and that the amount of the crushed catalyst to be mixed is restricted and the strength of the finished catalyst is remarkably deteriorated in preparing a catalyst which can not be put to practical use, if the amount of crushed catalyst added exceeds 50% by weight of the total catalyst.